Optical detection systems and methods of using the same

ABSTRACT

An optical detection system. The optical detection system includes a host node having (a) an optical source for generating optical signals, and (b) an optical receiver. The optical detection system also includes a plurality of fiber optic sensors for converting at least one of vibrational and acoustical energy to optical intensity information, each of the fiber optic sensors having: (1) at least one length of optical fiber configured to sense at least one of vibrational and acoustical energy; (2) a reflector at an end of the at least one length of optical fiber; and (3) a field node for receiving optical signals from the host node, the field node transmitting optical signals along the at least one length of optical fiber, receiving optical signals back from the at least one length of optical fiber, and transmitting optical signals to the optical receiver of the host node.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 61/338,466, filed on Feb. 18, 2010, and toU.S. Provisional Patent Application Ser. No. 61/367,515, filed on Jul.26, 2010, the contents of both of which are incorporated in thisapplication by reference.

TECHNICAL FIELD

This invention relates generally to the field of optical detectionsystems and, more particularly, to improved systems and methods foraccurately detecting presence in, and/or interference with, an area tobe monitored using fiber optics.

BACKGROUND OF THE INVENTION

Fiber optic sensing systems have been used in a number of applicationsincluding perimeter security, acoustic sensing, and leak detection.Examples of conventional fiber optic sensing systems include (1) modalinterference-based systems; (2) time-correlated Mach-Zehnderinterferometer-based systems; and (3) coherent Rayleighbackscattering-based systems. Each of these conventional systems suffersfrom certain deficiencies.

For example, modal interference-based systems provide very limitedinformation about an event such as the location and/or time of an event.Further, such systems have difficulty distinguishing between multiplesimultaneous events. Time-correlated Mach-Zehnder-based systems havedifficulty discerning continuous events (e.g., a pipe leak). CoherentRayleigh backscattering-based systems suffer from high interrogatorcosts and limited sensitivity. Further, certain of these conventionalsystems utilize photonics boxes located throughout an array that requireelectrical power to be provided locally, rendering such systemsimpractical for long distance applications such as border security.

Thus, a need exists for, and it would be desirable to provide, improvedoptical detection systems.

BRIEF SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes, the presentinvention provides, according to an exemplary embodiment, an opticaldetection system. The optical detection system includes a host nodeincluding (a) an optical source for generating optical signals, and (b)an optical receiver. The optical detection system also includes aplurality of fiber optic sensors for converting vibrational energy tooptical intensity information, each of the fiber optic sensorsincluding: (1) at least one length of optical fiber configured to sensevibrational energy; (2) a reflector at an end of the at least one lengthof optical fiber; and (3) a field node for receiving optical signalsfrom the host node, the field node transmitting optical signals alongthe at least one length of optical fiber, the field node receivingoptical signals back from the at least one length of optical fiber, andthe field node transmitting optical signals to the optical receiver ofthe host node.

According to another exemplary embodiment of the present invention, amethod of operating an optical detection system is provided. The methodincludes the steps of: (a) storing a plurality of predeterminedcharacteristics of events to be monitored using the optical detectionsystem in memory; (b) comparing a detected characteristic obtained fromthe optical detection system to the plurality of predeterminedcharacteristics stored in memory; and (c) determining if there is anacceptable level of matching between the detected characteristic and atleast one of the plurality of predetermined characteristics stored inmemory.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings. It is emphasizedthat, according to common practice, the various features of the drawingsare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1A is a block diagram illustrating an optical detection system inaccordance with an exemplary embodiment of the present invention;

FIG. 1B is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with a pipeline in accordance with anexemplary embodiment of the present invention;

FIG. 1C is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with a mine in accordance with an exemplaryembodiment of the present invention;

FIG. 1D is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with a fence line in accordance with anexemplary embodiment of the present invention;

FIG. 1E is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with underground monitoring in accordancewith an exemplary embodiment of the present invention;

FIG. 1F is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with underwater monitoring in accordance withan exemplary embodiment of the present invention;

FIG. 1G is a block diagram illustrating the optical detection system ofFIG. 1A utilizing point sensing transducers along each sensing zone inaccordance with an exemplary embodiment of the present invention;

FIG. 2 is a block diagram of a host node of an optical detection systemin accordance with an exemplary embodiment of the present invention;

FIG. 3 is a block diagram of a first field node of an optical detectionsystem in accordance with an exemplary embodiment of the presentinvention;

FIG. 4 is a block diagram of an intermediate field node of an opticaldetection system in accordance with an exemplary embodiment of thepresent invention;

FIG. 5 is a block diagram of a final field node of an optical detectionsystem in accordance with an exemplary embodiment of the presentinvention; and

FIG. 6 is a flow diagram illustrating a method of operating an opticaldetection system in accordance with an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to enable detection and classification of events in connectionwith a system or location to be monitored (e.g., a pipeline, a mine, afence line, an open area, a body of water, a perimeter, etc.), it isdesirable to have a high fidelity electronic representation of adisturbance (e.g., mechanical vibration, acoustic vibration, impact,intrusion, etc.). According to certain exemplary embodiments of thepresent invention, an optical detection system is provided whichutilizes interferometers with high linearity and dynamic range (e.g.,certain linearized Sagnac interferometers). The optical detectionsystems may also include a low noise, low distortion, optical receiver.

In certain more specific exemplary embodiments of the present invention,optical detection systems utilizing an integrated sensor array (e.g.,including a sensing cable divided into sensing zones which may bearranged to include a series of linearized Sagnac interferometers) formonitoring systems/locations are provided. Such optical detectionsystems may include a host node including an interrogation sub-systemand a signal processor.

Referring now to the drawings, in which like reference numbers refer tolike elements throughout the various figures that comprise the drawings,FIG. 1A illustrates an optical detection system 10. Optical detectionsystem 10 includes a plurality of fiber optic cables (i.e., opticalsensing cables) 400 a, 400 b, 400 c . . . , 400 n (which may be termedtransducers) configured into separate sensing zones 450, 455, 460 . . ., 499. Optical detection system 10 also includes a plurality of fieldnodes including a first field node 300; intermediate field nodes 500 a,500 b, etc.; and a final field node 600. Optical detection system 10also includes a lead cable 200 (e.g., a lead cable for telemetry ofprobe and return signals from each of the zones, a length of such leadcable being application dependent, with an exemplary lead cable being onthe order of meters to kilometers in length), a host node 100, and asignal processor 700. In the example shown in FIG. 1A, the opticaldetection system 10 includes a single host node 100, and a single firstfield node 300. Depending on the exact configuration of the opticaldetection system 10 (e.g., the number of sensing zones, the length ofthe cables covering each of the sensing zones, etc.), there may be aplurality of host nodes, first field nodes, etc., as is desired in thegiven application.

An exemplary operation of the configuration illustrated in FIG. 1A maybe summarized as follows. Host node 100 (which works in conjunction withsignal processor 700) generates optical signals and transmits thesignals along lead cable 200 to first field node 300 (e.g., where theelements and configuration of the optical detection system, includinglead cable 200, may be selected to minimize the lead cable sensitivityto vibration). As will be detailed below, part of the optical signalsfrom host node 100 (intended for use in monitoring sensing zone 450) aretransmitted through first field node 300 and along optical sensing cable400 a, are reflected back after reaching intermediate field node 500 a,where the reflected signals return along optical sensing cable 400 a andultimately return to host node 100 and signal processor 700 forprocessing. Another part of the optical signals from host node 100(intended for use in monitoring sensing zone 455) is transmitted throughfirst field node 300, along optical sensing cable 400 a, throughintermediate field node 500 a, along optical sensing cable 400 b, and isreflected back after reaching intermediate field node 500 b, where thereflected signals return along optical sensing cables 400 b, 400 a, andthe signals ultimately return to host node 100 and signal processor 700for processing. A similar process occurs for each subsequent sensingzone. As is clear in FIG. 1A, any number of desired subsequent sensingzones are contemplated (as indicated by the dotted line between zones460 and 499), with the final sensing zone 499 terminating with finalfield node 600.

FIGS. 1B-1F illustrate optical detection system 10 used in a variety ofsensing applications. In each of FIGS. 1B-1F, host node 100 and signalprocessor 700 are housed in a control room (i.e., control room 150, 160,170, 180, 190) or other desirable environment (e.g., a remote, stableenvironment). The fiber optic lead cable 200 runs from host node 100 tofirst field node 300.

FIG. 1B illustrates optical detection system 10 used to sensedisturbances (e.g., leaks, tampering events, etc.) along a pipeline 155,where each sensing zone 450, 455, 460 . . . 499 corresponds to a givenlength of pipeline 155. FIG. 1C illustrates optical detection system 10used to sense disturbances (e.g., presence of miners, voices, etc.)within a mine 165 (below the ground level 165 a and above the mine floor165 b), where each sensing zone 450, 455, 460 . . . 499 corresponds to agiven area of mine 165. FIG. 1D illustrates optical detection system 10used to sense disturbances (e.g., climbing, cutting, etc.) along a fenceline 175, where each sensing zone 450, 455, 460 . . . 499 corresponds toa given length of fence line 175. FIG. 1E illustrates optical detectionsystem 10 used to sense disturbances (e.g., walking, digging, tunneling,etc.) along an underground area 185 (e.g., a border area, a perimeterarea, etc. that is desired to be monitored), where each sensing zone450, 455, 460 . . . 499 corresponds to a given length of undergroundarea 185. FIG. 1F illustrates optical detection system 10 used to sensedisturbances (e.g., boats operating or divers breathing) along anunderwater area 195 (e.g., where the fiber cables may be buried in thearea 195 b beneath the ground beneath the water as illustrated in FIG.1F, or under the water area 195 a but above underground area 195 b),where each sensing zone 450, 455, 460 . . . 499 corresponds to a givenlength of underwater area 195.

Optical detection system 10 in FIGS. 1A-1F may solely utilize lengths ofoptical sensing cable 400 a, 400 b, etc. to sense disturbances; however,the present invention is not limited to that embodiment. For example,one or more point sensing transducers 50 may be integrated into each ofthe sensing zones. Such point sensing transducers 50 may be used tosense a disturbance at a specific “point” along a sensing cable segmentas opposed to general sensing anywhere along the sensing cable segment.Further, such point sensing transducers may include elements orstructure distinct from (and in addition to) the sensing cable segment.

In a specific example, FIG. 1G (where like reference numerals correspondto like elements from FIGS. 1A-1F) illustrates three accelerationtransducers 50 (e.g., part of an accelerometer) engaged with eachoptical sensing cable 400 a, 400 b, etc. A detailed view is provided ofan exemplary acceleration transducer 50 that includes a fixed member 50a configured to be secured to a body of interest 50 d and a movablemember 50 b. Transducer 50 includes a spring member 50 c between movablemember 50 b and fixed member 50 a. A portion of optical fiber (e.g.,part of optical sensing cable 400 a) is wrapped for one or more turns“T” around fixed member 50 a and movable member 50 b (e.g., where theturns “T” of the optical fiber span the spring member 50 c). Thus, whena disturbance occurs the movable member 50 b moves (e.g., along at leastone range of motion such as motion axis “m”) with respect to fixedmember 50 a, where such movement causes a change in strain to theoptical fiber of the optical sensing cable. As will be appreciated bythose skilled in the art, use of one or more transducers 50 may provideimproved sensing of disturbances. Such point sensing transducers 50 maybe used in connection with any configuration of the present inventionincluding but not limited to the embodiments shown and described inFIGS. 1A-1F.

Details of the elements of an exemplary optical detection system 10 (inany of FIGS. 1A-1G) is now described. Referring to FIG. 2, host node 100includes one or more optical sources 110 (e.g., LED sources such assuperluminescent light emitting diodes, edge emitting light emittingdiodes, other light emitting diode sources, lasers, etc.) within anenclosure 112. According to an exemplary embodiment of the presentinvention, optical source 110 may be a broadband optical source operatedin a continuous wave (CW) mode. Optical source 110 is controlled by asource control circuit 111. In the exemplary embodiment now described(described and illustrated in connection with four sensing zones),optical source 110 is connected via an optical cable 120 to a 1×4splitter (such as a 1×4 or 4×4 fiber optic coupler or an integratedoptic splitter) labeled as optical coupler 130. Optical coupler 130divides the light intensity output from optical source 110 into foursignals along respective fibers 140 a, 140 b, 140 c, and 140 d (e.g.,four substantially equal intensity signals) that are each output to arespective input lead of a corresponding optical circulator 150 a, 150b, 150 c, and 150 d (e.g., identical optical circulators 150 a, 150 b,150 c, and 150 d). Output signals are provided along each of fibers 160,161, 162, 163 within fiber optic lead cable 200 from a respective one ofoptical circulators 150 a, 150 b, 150 c, and 150 d.

As provided above, according to certain exemplary embodiments of thepresent invention, linearized Sagnac interferometers are utilized. Aswill be appreciated by one skilled in the art, in order to provide alinearized Sagnac interferometer, the architecture of a traditional loopconfiguration Sagnac interferometer (e.g., typically used to senserotation) is modified (e.g., folded) to allow measurements of phaseperturbations along an optical fiber in a non-looped configuration, forexample, by incorporation of a 1×2 fiber optic coupler. Referring againto FIG. 2 (and FIG. 3), light output from host node 100 travels alongeach of fibers 160, 161, 162, and 163 within lead cable 200 which isconnected to first field node 300. First field node 300 includes anenclosure 310 which houses a series of components.

In FIG. 3, fiber 160 is connected to an input/output lead 315 of anoptical circulator 320. A lead 317 of optical circulator 320 isconnected to a lead 322 of an optical coupler 330 (e.g., a 3×3 fiberoptic coupler 330). A lead 319 of optical circulator 320 is connected toa lead 324 of optical coupler 330.

A lead 332 of optical coupler 330 is connected to a lead 335 of a delaycoil 340. The fiber optic delay coil 340 has a length of, for example,at least twice the length of the zone 450 of an optical fiber 380 inoptical sensing cable 400 a where the midpoint of the sensing loop(e.g., from one output leg of the 3×3 coupler to another) including thesensing optical fiber 380 “unfolded” is within the enclosure 310 formaximum sensitivity. A lead 341 of delay coil 340 is connected to a lead342 of an optical coupler 360 (e.g., a 2×2 fiber optical coupler 360).

A lead 334 of optical coupler 330 is connected to a lead 354 of adepolarizer 350. A lead 326 of optical coupler 330 is tied off and/orthe end crushed to minimize light that is reflected back into opticalcoupler 330. Similarly, a lead 336 of optical coupler 330 is tied offand/or the end crushed to minimize light that is reflected back intooptical coupler 330.

Depolarizer 350 significantly reduces polarization-induced signalfading, allowing inexpensive single mode fiber to be used for all of theoptical components and cable fibers rather than costlypolarization-maintaining fiber. Depolarizer 350 may be one of severalcommercially available depolarizers, such as, for example, arecirculating coupler (single or multiple stage) or a Lyot Depolarizer.A lead 352 of depolarizer 350 is connected to a lead 366 of opticalcoupler 360. A lead 362 of optical coupler 360 is connected to fiber 380in optical sensing cable 400 a. A lead 364 of optical coupler 360 istied off and/or the end crushed to minimize light that is reflected backinto optical coupler 360. Although one example for optical coupler 360is a 2×2 fiber optic coupler, optical coupler 360 is not limited to thatembodiment. For example, a 1×2 fiber optic coupler may be used insteadof a 2×2 fiber optic coupler 360, thereby obviating the tying off ofsecond output lead 364.

Fibers 161, 162, and 163 in lead cable 200 are connected to fibers 370,372, and 374 in field node 300, respectively. These are pass-throughfibers not actively used in first field node 300, but rather to be usedin connection with sensing in other nodes. Fibers 370, 372, and 374 areconnected to fibers 382, 384, and 386 in optical sensing cable 400 a,respectively. Fiber 380 in optical sensing cable 400 a is used forsensing within zone 450. Fiber 380 in optical sensing cable 400 a (whichhad been used for sensing in zone 450) is attached to a fiber 580 inintermediate field node 500 a (see FIG. 4). Fiber 580 is connected to areflector 581 (e.g., broadband reflector 581). Disturbances alongsensing cable 400 a cause small changes in the length of fiber 380.These changes cause non-reciprocal changes in the phase of the lighttravelling through the Sagnac interferometer.

An exemplary operation of first field node 300 shown in FIG. 3 (andpartially in FIG. 4) is now provided. An optical signal (i.e., lightfrom host node 200 entering first field node 300) propagates along fiber160 to lead 315 and enters port 2 of optical circulator 320, and thenexits port 3 of optical circulator 320 through lead 317, and thenpropagates along lead 322 (a length of optical fiber) to optical coupler330. Optical coupler 330 divides the light into optical signals alongtwo counterpropagating paths: a first path of the divided light extendsfrom lead 332 to delay coil 340 along lead 335, and then from lead 341to optical coupler 360 through lead 342; a second path of the dividedlight extends from lead 334 to depolarizer 350 through lead 354, andthen from lead 352 to optical coupler 360 through lead 366. Thus, thelight along the first path is delayed with respect to the light alongthe second path by a time approximately proportional to the length ofdelay coil 340. The two counterpropagating optical signals recombine atoptical coupler 360, and the recombined optical signal exits opticalcoupler 360 along lead 362, and then travels along fiber 380 (forsensing within zone 450) of optical sensing cable 400 a. The recombinedoptical signal enters field node 500 a on fiber 380, and propagatesalong lead 580 to reflector 581, and is then reflected back along fiber380 to first field node 300. This reflected signal is divided into twooptical signals by optical coupler 360, where each of the opticalsignals travels along a counterpropagating path and recombinescoherently at optical coupler 330. The result of the optical signalsrecombining at optical coupler 330 is that the recombined light has anintensity output proportional to the phase perturbation from theoriginal disturbance along fiber 380 within optical sensing cable 400 a.This optical signal (having a variable intensity) is output from opticalcoupler 330 along lead 324 (i.e., fiber 324) and then along lead 319into port 1 of optical circulator 320. This optical signal propagatesfrom port 1 to port 2 of optical circulator 320, and then along lead 315to fiber 160 of lead cable 200. The signal is transmitted along fiber160 of lead cable 200 to the interrogator of host node 100.

Referring now to FIG. 4, fibers 384 and 386 in optical sensing cable 400a are connected to fibers 570, 572 in intermediate field node 500 a,respectively. These are pass-through fibers not actively used inintermediate field node 500 a, but rather to be used in connection withsensing in other nodes. Fibers 570, 572 are connected to fibers 584, 586in optical sensing cable 400 b, respectively. Fiber 582 in opticalsensing cable 400 b is used for sensing within zone 455.

Fiber 382 from optical sensing cable 400 a is connected to aninput/output lead 515 of an optical circulator 520. The lead 517 ofoptical circulator 520 is connected to a lead 522 of an optical coupler530 (e.g., a 3×3 fiber optic coupler 530). A lead 519 of opticalcirculator 520 is connected to a lead 524 of optical coupler 530.

A lead 532 of optical coupler 530 is connected to lead 535 of a delaycoil 540. The fiber optic delay coil 540 has a length of, for example,at least twice the length of the zone 455 of optical fiber 582 in fiberoptic sensing cable 400 b where the midpoint of the sensing loop (e.g.,from one output leg of the 3×3 coupler to another), including thesensing optical fiber 582 “unfolded” is within the enclosure 510 formaximum sensitivity. A lead 541 of delay coil 540 is connected to a lead542 of an optical coupler 560 (e.g., a 2×2 fiber optic coupler 560).

A lead 534 of optical coupler 530 is connected to a lead 554 of adepolarizer 550. A lead 526 of optical coupler 530 is tied off and/orthe end crushed to minimize light that is reflected back into opticalcoupler 530. Similarly, a lead 536 of optical coupler 530 is tied offand/or the end crushed to minimize light that is reflected back intooptical coupler 530. A lead 552 of depolarizer 550 is connected to alead 566 of optical coupler 560. A lead 562 of optical coupler 560 isconnected to fiber 582 in optical sensing cable 400 b. A lead 564 ofoptical coupler 560 is tied off and/or the end crushed to minimize lightthat is reflected back into optical coupler 560. Although an exemplaryoptical coupler 560 is a 2×2 fiber optic coupler, the optical coupler560 is not limited to that embodiment. For example, a 1×2 fiber opticcoupler may be used instead of a 2×2 fiber optic coupler 560, therebyobviating the tying off of lead 564.

An exemplary operation of field node 500 a shown in FIG. 4 is nowprovided. An optical signal (i.e., light from host node 200 enteringfield node 500 a) propagates along fiber 382 to lead 515 and enters port2 of optical circulator 520, and then exits port 3 of optical circulator520 through lead 517, and then propagates along lead 522 (a length ofoptical fiber) to optical coupler 530. Optical coupler 530 divides thelight into optical signals along two counterpropagating paths: a firstpath of the divided light extends from lead 532 to delay coil 540 alonglead 535, and then from lead 541 to optical coupler 560 through lead542; a second path of the divided light extends from lead 534 todepolarizer 550 through lead 554, and then from lead 552 to opticalcoupler 560 through lead 566. Thus, the light along the first path isdelayed with respect to the light along the second path by a timeapproximately proportional to the length of delay coil 540. The twocounterpropagating optical signals recombine at optical coupler 560, andthe recombined optical signal exits optical coupler 560 along lead 562,and then travels along fiber 582 (for sensing within zone 455) ofoptical sensing cable 400 b. The recombined optical signal enters fieldnode 500 b (see FIGS. 1A-1G) on fiber 582, and is reflected back (usinga reflector in field node 500 b similar to reflector 581 in field node500 a) along fiber 582 to field node 500 a. This reflected signal isdivided into two optical signals by optical coupler 560, where each ofthe optical signals travels along a counterpropagating path andrecombines coherently at optical coupler 530. The result of the opticalsignals recombining at optical coupler 530 is that the recombined lighthas an intensity output proportional to the phase perturbation from theoriginal disturbance along fiber 582 within optical sensing cable 400 b.This optical signal (having a variable intensity) is output from opticalcoupler 530 along lead 524 (i.e., fiber 524) and then along lead 519into port 1 of optical circulator 520. This optical signal propagatesfrom port 1 to port 2 of optical circulator 520, and then along lead 515to fiber 382/pass through fiber 370 to fiber 161 of lead cable 200. Thesignal is transmitted along fiber 161 of lead cable 200 to theinterrogator of host node 100.

The pattern of field nodes 500 a, 500 b, etc. and optical sensing cables400 a, 400 b, etc. is repeated, as desired, and utilizing the number ofavailable optical fibers within the cable. Other system level topologies(e.g., branching, bi-directional/redundancy, etc.) are contemplatedusing this modular approach. Each optical sensing cable 400 a, 400 b,etc. may be used to provide an acoustically independent sensing zone.FIG. 5 illustrates final field node 600 including an enclosure 610 forreceiving final optical sensing cable 400 n. Optical sensing cable 400 nincludes a fiber 680 which is connected to a reflector 681 (e.g.,broadband reflector 681).

Referring back to FIG. 2, optical intensity signals proportional to thephase perturbations within each zone (e.g., due to mechanical oracoustic vibrations sensed) are returned to host node 100 (which may beconsidered an interrogator) by way of fibers 160, 161, 162, and 163 andthen through circulators 150 a, 150 b, 150 c, and 150 d after conversionfrom a phase signal to an intensity signal at coupler 330 or 530, etc.Circulators 150 a, 150 b, 150 c, and 150 d are configured to behave insuch as way as to allow signals from fiber 160 to pass through to afiber 174, for signals from fiber 161 to pass through to a fiber 173,for signals from fiber 162 to pass through to a fiber 172, and forsignals from fiber 163 to pass through to a fiber 171. However, thecirculators 150 prevent light from passing from: fiber 160 or fiber 174to fiber 140 a; fiber 161 or fiber 173 to fiber 140 b; fiber 162 orfiber 172 to fiber 140 c; and fiber 163 or fiber 171 to fiber 140 d,etc. Light from fiber 174 is converted to an electrical current signalat a photodetector 175. Likewise, light from fiber 173 is converted toan electrical current signal at a photodetector 176, light from fiber172 is converted to an electrical current signal at a photodetector 177,and light from fiber 171 is converted to an electrical signal at aphotodetector 178. The electrical signals converted by photodetectors175, 176, 177, and 178 may be very low noise signals, with dark currentless than about 0.5 nA.

The outputs of photodetectors 175, 176, 177, and 178 are then amplifiedusing transimpedance amplifiers 180 (e.g., amplifiers of very lowdistortion (less than −90 dB), high gain bandwidth (on the order of500-2,000 MHz), and noise less than 1 nV/√Hz (such as the model AD8099,produced by Analog Devices, Inc.)). Multiple stages of furtheramplification may follow each transimpedance amplifier 180 as is knownby those skilled in the state of the art. The electrical outputs ofamplifiers 180 are filtered using filters 181. Use of high qualityphotodetectors, amplifiers, and filters desirably produces signals withfidelity sufficient for advanced signal processing desired for robustclassification of detected events and alarm generation (or otherindications based on mechanical/acoustic vibration) without falsealarms. The signals output from filters 181 are sampled by A/Dconverters (ADCs) 182. The sampled electrical signals from ADCs 182 arereceived by one or more Field Programmable Gate Arrays (FPGAs) 184.

FPGAs 184 may be configured to perform high speed signal pre-processing.Such FPGAs 184 are typically used to perform filtering and Fast FourierTransforms (FFTs) of the sampled data from each zone to determine theinstantaneous spectrum of the disturbance(s) along each zone. Furtherprocessing is performed by a microprocessor 186 as shown in FIG. 2.Communication with outside security system processors and otherperipheral devices is accomplished with an interface chip 188. Interfacechip 188 may be for example, an RS-232 interface chip or a USBtransceiver.

An exemplary signal processing sequence is accomplished as follows. Fromeach sensing zone (e.g., zone 450, zone 455, zone 460, etc.), ADCs 182digitize a set of data samples (e.g., at an exemplary rate of 8192samples per second). In such an example, FPGA 184 performs a 8192 sampleFFT to produce spectra, which are output to the microprocessor 186.Microprocessor 186 groups the spectra output from FPGA 184 into datawindows (e.g., on the order of 0.25 seconds).

In such an example, a series of spectral masks are created by processingsignals generated during the introduction of known events (where suchevents may be configured depending upon the application). In a pipelinedetection application such an event may be a hostile/alarm event such asdrilling of a portion of the pipeline, cutting of a portion of thepipeline, fluid leakage from a portion of the pipeline, etc. Spectragenerated by FPGA 184 during these events are saved, for example, in adatabase, a look-up table, or other data storage techniques. Each ofthese spectral masks is further modified to create a dynamic signalthreshold. The spectrum of the received data within each data window iscompared to the signal thresholds. A persistence requirement isestablished that requires “m” spectra to exceed a spectral mask forevery “n” contiguous time windows which, when true, is reported as analarm condition. The use of persistence helps minimize false alarms dueto instantaneous (non-alarm) events of high energy.

The dynamic threshold is continually updated wherein a single value iscalculated for each frequency band within a spectrum by summing thevalues of a common frequency band from all of the zones in anenvironmental zone (where the environmental zone is a set of realsensing zones artificially grouped by the user). These values areintegrated over a user-defined time span. This dynamic threshold is usedto compensate for non-instantaneous environmental effects impactingmultiple zones (e.g., lasting on the order of seconds to hours), such asrain, hail, highway traffic, trains, etc. The shorter this time span ofthe dynamic threshold integration, the more rapidly the dynamicthreshold changes. The longer this time span, the more the dynamicthreshold response is damped. In addition, the amount that any oneinstantaneous spectra can bias the dynamic threshold can also be limitedto prevent single events (such as an impact from a falling tree branch)from having an undue impact upon the threshold.

Electrical outputs from filters 181 in host node 100 may be combined anddistinguished by use of a multiplexer, switch, or other appropriatemechanism 1000 to an amplifier or line driver 1010 to provide an audiooutput of any zone desired by a user. Providing an audible outputenhances the functionality of the optical detection system 10 byenabling the user to hear the detected events as alarms are generated.

The present invention also includes methods of operating opticaldetection systems such as the optical detection systems 10 illustratedand described in connection with FIGS. 1A-1G and FIGS. 2-5. FIG. 6illustrates an example of such a method implemented in a closed-loopfashion. At step 600, a plurality of predetermined characteristics ofevents to be monitored using an optical detection system are stored inmemory. By “predetermined” is meant determined beforehand, so that thepredetermined characteristic must be determined, i.e., chosen or atleast known, in advance of some event such as implementation of themethod. Depending upon the application of the optical detection system,such events (and therefore, the predetermined characteristics of suchevents) may vary broadly. For example, in a pipeline detection system,exemplary events may include a pipe leak, a pipe being cut by a saw, apipe being struck by an object (e.g., a hammer), etc. Further still, thecharacteristics of the events may vary broadly. As provided above, sucha characteristic may be spectra or a spectrum of a known event. Such aspectrum may be an energy profile over a plurality of frequencies, etc.

In one specific example, in order to provide the characteristics at step600, a number of substeps are completed. In a first substep, a windowingfunction (such as a Hanning function or Beckman function) is applied toa sampled set of data points within a series of time windows during aseries of known events (e.g., leaks, walking, cutting, etc.). In asecond substep, a spectrum is created by applying a Fast FourierTransform (FFT) on the windowed data. In a third substep, the spectrumis scaled in a way to include a population of system responses to aseries of similar events (e.g., in such a way as to minimize falsealarms) to create a spectral mask. In a fourth substep, the resultantspectral mask is associated with each event and is stored in a datastructure (e.g., a database or other similarly retrievable structure).

At step 602, a detected characteristic obtained from the opticaldetection system (e.g., obtained from the host node by processing ofoptical intensity information received from the various field nodes) iscompared to the plurality of predetermined characteristics stored inmemory. Referring again to the spectra example described above, step 602may include two substeps. In a first substep, windowed samples of dataare acquired during normal operation, and spectra of this data aregenerated as a function of time. Then, in a second substep, the spectragenerated during normal operation are compared to those previouslyassociated with alarm events and stored (e.g., compared to thecharacteristic provided in step 600).

At step 604, a determination is made as to whether there is anacceptable level of matching between the detected characteristic fromstep 602 and at least one of the plurality of predeterminedcharacteristics stored in memory in step 600. If there is no suchacceptable level of matching (i.e., a “No” answer at step 604), then theprocess returns to step 602 and further comparisons are made withupdated data. If there is such an acceptable level of matching (i.e., a“Yes” answer at step 604) then an alarm may be generated at step 608.

As will be appreciated by those skilled in the art, certain types ofevents may be of a momentary nature, and a momentary match (i.e., amomentary acceptable level of matching at step 604) may suffice togenerate an alarm at step 608. However, other types of events may be ofsuch a type where it is appropriate to confirm that the event continuesfor a predetermined period of time. In such a case, even if there issuch an acceptable level of matching (i.e., a “Yes” answer at step 604)at step 602, the process may not immediately generate an alarm, butrather may proceed to step 606 where a determination is made as towhether the acceptable level of matching is present for a predeterminedperiod of time (e.g., or apply a persistence test to the processedoperational data to see if it exceeds an alarm threshold, where suchthreshold may be the predetermined period of time, or some otherthreshold). If the answer at step 606 is “Yes,” then an alarm isgenerated at step 608. If the answer at step 606 is “No,” then theprocess proceeds to step 602 for continued monitoring. The step 606 ofdetermining if the acceptable level of matching is present for apredetermined period of time can be accomplished in a closed loopfashion wherein a counter is updated for each incremental time periodduring which there is an acceptable level of matching.

Although the present invention has been described in connection withcertain exemplary applications (e.g., pipeline monitoring, minemonitoring, fence line monitoring, underground monitoring, underwatermonitoring, etc.) it is not limited to those applications. The opticaldetection system may be applied to any of a number of applications asdesired by the user.

The optical fibers and cables illustrated and described herein may bearranged in any desired configuration. For example, each of the fibersmay be provided in a single length between elements, or in multiplelengths, as desired. In a specific example, fiber 160 in FIG. 3 connectsto port 2 of optical circulator 320 through lead 315; however, it isunderstood that lead 315 may be part of fiber 160 if desired. Likewise,port 3 of optical circulator 320 and optical coupler 330 are connectedthrough leads 317 and 322; however, it is understood that leads 317 and322 may be part of the same length of optical fiber if desired.

Although the present invention has been described in connection withcertain exemplary elements (e.g., the elements illustrated and describedin connection with FIGS. 2-5) it is not limited to those elements. Theoptical detection system may use any of a number of types of componentswithin the scope and spirit of the claims.

Although illustrated and described above with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. An optical detection system comprising: a host nodeincluding (a) an optical source for generating optical signals, and (b)an optical receiver; and a plurality of fiber optic sensors forconverting at least one of vibrational and acoustical energy to opticalintensity information, each of the fiber optic sensors including: (1) atleast one length of optical fiber configured to sense at least one ofvibrational and acoustical energy; (2) a reflector at an end of the atleast one length of optical fiber; and (3) at least one field node forreceiving optical signals from the host node, the field nodetransmitting optical signals along the at least one length of opticalfiber, the field node receiving optical signals back from the at leastone length of optical fiber, and the field node transmitting opticalsignals to the optical receiver of the host node.
 2. The opticaldetection system of claim 1 wherein the optical source includes at leastone of a light emitting diode and a laser.
 3. The optical detectionsystem of claim 1 wherein the optical source includes at least one of asuperluminescent light emitting diode and an edge emitting lightemitting diode.
 4. The optical detection system of claim 1 wherein theoptical source transmits optical signals as light energy in a continuouswave (CW) mode.
 5. The optical detection system of claim 1 wherein thefield node includes a linearized Sagnac interferometer.
 6. The opticaldetection system of claim 1 wherein the linearized Sagnac interferometerincludes a 3×3 fiber optic coupler, a delay coil of optical fiber, adepolarizer, and a 2×2 fiber optic coupler.
 7. The optical detectionsystem of claim 6 wherein an output of the optical source is connectedto a first input lead of the 3×3 fiber optic coupler, and wherein asecond input lead of the 3×3 fiber optic coupler is connected to theoptical receiver of the host node.
 8. The optical detection system ofclaim 7 wherein a third input lead of the 3×3 fiber optic coupler isconfigured such that it does not support guiding light, and as such,reflected light can not travel along the third input lead back to the3×3 fiber optic coupler, and wherein a lead of the delay coil isconnected to an output lead of the 3×3 fiber optic coupler, and anotherlead of the delay coil is connected to an input lead of the 2×2 fiberoptic coupler.
 9. The optical detection system of claim 7 furthercomprising a lead cable between the host node and a first of theplurality of fiber optic sensors closest in proximity to the host node,the lead cable being connected to the field node of the first of theplurality of fiber optic sensors.
 10. The optical detection system ofclaim 9 wherein a connection between the output of the optical sourceand the first input lead of the 3×3 fiber optic coupler, and aconnection between the second input lead of the 3×3 fiber optic couplerand the optical receiver of the host node, are provided by the leadcable.
 11. The optical detection system of claim 9 wherein a connectionbetween the output of the optical source and the first input lead of the3×3 fiber optic coupler is provided through a first optical circulator,and a connection between the second input lead of the 3×3 fiber opticcoupler and the optical receiver of the host node is provided through asecond optical circulator.
 12. The optical detection system of claim 1wherein the host node is configured to receive and interpret the opticalintensity information from the plurality of fiber optic sensors, thehost node being configured to (1) collect and save a set of data samplesover a specified time window; (2) perform a Fourier Transform on the setof data samples within each time window to generate a series of spectrain time; (3) generate a spectral mask representing a vibration spectrumof a predetermined plurality of events; (4) compare spectra of theoptical intensity information received from the plurality of fiber opticsensors to the spectral mask to ascertain whether the received opticalintensity information exceeds the spectral mask within a time window.13. The optical detection system of claim 1 wherein the at least onefield node converts phase information received from the at least onelength of optical fiber into intensity information.
 14. The opticaldetection system of claim 1 wherein each of the plurality of fiber opticsensors includes at least one transducer, the at least one transducerincluding (1) a fixed member configured to be secured to a body ofinterest and (2) a movable member, the at least one length of opticalfiber wrapped for at least one turn around the fixed member and themovable member.
 15. A pipeline detection system comprising: a host nodein the vicinity of, or remote from, a pipeline to be monitored, the hostnode including an optical source for generating optical signals, and anoptical receiver; and a plurality of fiber optic sensors local to thepipeline to be monitored, the plurality of fiber optic sensors forconverting at least one of vibrational and acoustical energy to opticalintensity information, each of the fiber optic sensors including: (1) atleast one length of optical fiber affixed along a portion of thepipeline to sense at least one of vibrational and acoustical energy; (2)a reflector at an end of the at least one length of optical fiber; and(3) a field node for receiving optical signals from the host node, thefield node transmitting optical signals along the at least one length ofoptical fiber, the field node receiving optical signals back from the atleast one length of optical fiber, and the field node transmittingoptical signals to the optical receiver of the host node.
 16. A minedetection system comprising: a host node remote from a mine to bemonitored, the host node including an optical source for generatingoptical signals, and an optical receiver; and a plurality of fiber opticsensors local to the mine to be monitored, the plurality of fiber opticsensors for converting acoustical energy to optical intensityinformation, each of the fiber optic sensors including: (1) at least onelength of optical fiber affixed along a portion of the mine to senseacoustical energy within the mine; (2) a reflector at an end of the atleast one length of optical fiber; and (3) a field node for receivingoptical signals from the host node, the field node transmitting opticalsignals along the at least one length of optical fiber, the field nodereceiving optical signals back from the at least one length of opticalfiber, and the field node transmitting optical signals to the opticalreceiver of the host node.
 17. A fence detection system comprising: ahost node in the vicinity of, or remote from, a fence to be monitored,the host node including an optical source for generating opticalsignals, and an optical receiver; and a plurality of fiber optic sensorsmounted along the fence to be monitored, the plurality of fiber opticsensors for converting vibrational energy to optical intensityinformation, each of the fiber optic sensors including: (1) at least onelength of optical fiber affixed along a portion of the fence to sensevibrational energy; (2) a reflector at an end of the at least one lengthof optical fiber; and (3) a field node for receiving optical signalsfrom the host node, the field node transmitting optical signals alongthe at least one length of optical fiber, the field node receivingoptical signals back from the at least one length of optical fiber, andthe field node transmitting optical signals to the optical receiver ofthe host node.
 18. An underground detection system comprising: a hostnode in the vicinity of, or remote from, an underground area to bemonitored, the host node including an optical source for generatingoptical signals, and an optical receiver; and a plurality of fiber opticsensors local to the underground area to be monitored, the plurality offiber optic sensors for converting vibrational energy to opticalintensity information, each of the fiber optic sensors including: (1) atleast one length of optical fiber buried along a portion of theperimeter to sense vibrational energy; (2) a reflector at an end of theat least one length of optical fiber; and (3) a field node for receivingoptical signals from the host node, the field node transmitting opticalsignals along the at least one length of optical fiber, the field nodereceiving optical signals back from the at least one length of opticalfiber, and the field node transmitting optical signals to the opticalreceiver of the host node.
 19. An underwater detection systemcomprising: a host node in the vicinity of, or remote from, a body ofwater to be monitored, the host node including an optical source forgenerating optical signals, and an optical receiver; and a plurality offiber optic sensors within or under the body of water to be monitored,the plurality of fiber optic sensors for converting acoustic energy tooptical intensity information, each of the fiber optic sensorsincluding: (1) at least one length of optical fiber suspended in thewater or buried under the water to sense acoustic energy; (2) areflector at an end of the at least one length of optical fiber; and (3)a field node for receiving optical signals from the host node, the fieldnode transmitting optical signals along the at least one length ofoptical fiber, the field node receiving optical signals back from the atleast one length of optical fiber, and the field node transmittingoptical signals to the optical receiver of the host node.
 20. A methodof operating an optical detection system comprising: (a) storing aplurality of predetermined characteristics of events to be monitoredusing the optical detection system in memory; (b) comparing a detectedcharacteristic obtained from the optical detection system to theplurality of predetermined characteristics stored in memory; and (c)determining if there is an acceptable level of matching between thedetected characteristic and at least one of the plurality ofpredetermined characteristics stored in memory.
 21. The method of claim20 further comprising the step of (d) generating an alarm condition ifit is determined that there is an acceptable level of matching at step(c).
 22. The method of claim 20 wherein, if it is determined that thereis an acceptable level of matching at step (c), the method furthercomprises the step of (d) determining if the acceptable level ofmatching is present for a predetermined period